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At age 14, she suffered a slow and painful death - raped, drugged and left to die in a sweltering attic, court records say.

When those actions didn't result in her death, she was strangled, US prosecutors say. Then her body was packed in cat litter and left in the same attic for months. Police would later find her dismembered remains in rural Pennsylvania.

Officials say her gruesome death was carried out by a 41-year-old woman she had known as her mother, who adopted the teenager and her younger brother, and the mother's boyfriend. Sara Packer and Jacob Sullivan were charged on Sunday with several crimes, including homicide, kidnapping, endangering the welfare of a child and abuse of corpse. Sullivan who, police say, raped Grace in the attic, is facing additional sexual assault charges.

Sara Packer and Jacob Sullivan have been charged with a number of crimes.

Sara Packer and Jacob Sullivan have been charged with a number of crimes.

Bucks County District Attorney Matthew Weintraub said the teen's brutal killing was the result of a months-long plan - "a rape-murder fantasy" shared by her adoptive mother and her boyfriend. "Unfortunately, Grace Packer was a disposable child to these people," Weintraub said in a news release. "Who will now speak for Grace Packer? We will."

Authorities say that about a week before they were arrested on Saturday, Packer and Sullivan were rushed to the hospital after they tried to kill themselves by overdosing on pills. A probable cause affidavit said the two had taken a suicide pact.

The arrests and charges come after Sullivan confessed, in detail, to hospital staff about the teenager's death, the affidavit said.

Sullivan told hospital employees and investigators that he and Packer began plotting the teen's murder in autumn of 2015 and carried it out on July 8, 2016, in their house in Quakertown, Pennsylvania, a borough in Bucks County about 65km north of Philadelphia. Sullivan struck Grace several times in the face, splitting her lip, the affidavit said, and then he and Packer took the teen to the attic on the third floor.

At some point, Packer left to get some medicine to sedate Grace, Sullivan told investigators. The two later tied and gagged the teen and left her in a cedar closet, believing she would eventually die from the effects of the assault, the drugs and the excessive heat in the attic, the affidavit said.

At about 3am the next morning - more than 12 hours later - they checked on Grace and found that she was still alive and conscious. That's when Sullivan strangled her to death, admitting to investigators that it was "more physical and took much longer than he expected," the affidavit said.

Prosecutors said the two packed the teen's remains in cat litter to mask the odour. For the next three months, Grace's body would remain in the attic.

Packer is also accused of lying to police about her daughter's whereabouts.

A few days after the teen was believed to have been killed, on July 11, Packer told police that Grace had stolen $300 from her and had run away from home, the affidavit said. She claimed that they had a fight the night of July 8 because she didn't allow the teen to go to a friend's house. Grace was gone when she checked on her the following morning, Packer told police, adding that her daughter has been gone for "days at a time" before.

But investigators found inconsistencies in Packer's statements. For instance, in September, Packer told a detective investigating the missing persons report that she had notified relatives about her daughter's disappearance, the affidavit said. But a North Carolina relative, who was Grace's caretaker when she stayed there in 2015, said she didn't know the teen was missing, according to the affidavit. Other family members told investigators that they weren't told about Grace's disappearance until later.

Investigators also noticed oddities in Packer's behaviour. In the months after Packer filed a missing person's report, detectives were largely unable to get a hold of her. A frequent Facebook user with a habit of posting about insignificant incidents, such as having a headache, Packer never once posted anything about her daughter's disappearance, according to investigators.

After detectives visited the Quakertown home in mid-October for a follow-up investigation, Packer and Sullivan became concerned that investigators would discover Grace's body, which was still in the attic. They dismembered the teen and drove to a remote area where they dumped the remains, the affidavit said.

Later that month, Pennsylvania State Police troopers responded to a call about human remains found in rural Bear Creek Township in Luzerne County, about 70 miles from the couple's home.

After searching the house, detectives found a receipt for a bow saw and two blades believed to have been used to dismember Grace's body. A surveillance video showed Packer buying the items at a local tractor supply store. Records of Packer's debit card purchases also revealed that she had bought Diphenhydramine tablets and Tylenol capsules at a local Target store on July 8, the same day that Grace was believed to have been raped and drugged, the affidavit said.

On December 30, authorities said, Packer and Sullivan tried to commit suicide by overdosing on pills. Police said they believed that the two had agreed to kill themselves. At that time, they had moved to an apartment in a neighbouring town. A woman who had been living with the couple called emergency services.

"I don't know if you have watched the news lately ... our ... someone we were involved with was recently ... It's a big mess, it's a big mess, and I don't really know how to explain it...but...oh my God," Katherine Albright told the dispatcher, according to the affidavit.

Police also said they found a suicide note in which Sullivan appears to try to explain his innocence to his children. It read:

"I love you all so much. You are the only people that I have always been able to count on. I'm sorry that I am taking the coward's way out, but I don't have any strength left in me.

People want to judge and lie and break me down. They have.

I can't exist with Sara in jail and those f-- lying pigs and the whore media have made it impossible for us to live.

They don't care how many lives or laws they break. I know you will always know that we had nothing to do with this no mater what lies they tell. I'm sorry to leave you. Remember all I've taught you. Be brave, stand tall and do your best to be stronger than I was."

Bucks County District Attorney Matt Weintraub, right, with Chief of Trials of the District Attorney's office, Jennifer Schorn. Photo / AP

Bucks County District Attorney Matt Weintraub, right, with Chief of Trials of the District Attorney's office, Jennifer Schorn. Photo / AP

But about a week later, investigators were told that Sullivan had confessed to hospital staff and to his family that he had killed Grace and that Packer was his accomplice. Sullivan was arrested on Saturday, and Packer was arrested about three hours later, according to the district attorney's office. The two were arraigned separately on Sunday. A district judge denied them bail.

"I'm sorry for what I did," Sullivan told reporters as he was led into the courthouse, according to a news release from the district attorney's office.

Weintraub, the district attorney, called the crimes "heinous" and "depraved," according to the Philadelphia Inquirer. After Packer's arraignment, Weintraub told the paper that the teen's death "shakes one's belief in the goodness of humanity."

"The question is: How could any mother do this to a child?" he told the paper. "I'm bereft. I don't have an answer."

Grace's 12-year-old biological brother is with other family members, Weintraub told the Inquirer.

Packer and Sullivan are scheduled for a preliminary hearing on January 20. It's not immediately clear who their attorneys are.